Joint Replacements, Phantom Limbs and Warning Lights

I work with chronic pain patients quite a bit in the clinic. Using modern neuroscience education and graded motor imagery we are able to cure pain conditions that have been classified as untreatable. I have written previous posts on these topics here and here, and would encourage you to check that out if you are not familiar. Although I like working with chronic pain patients, the majority of people I see are orthopedic patients (knee, back, shoulder etc.).

One particular gentleman came to me a few months ago after undergoing a total knee replacement. His rehabilitation has progressed very well. He has full range of motion and strength, and is back to doing most of his normal activities. However, he has had some lingering pain that continues to limit him. He asked, “How come this darn pain in the knee won’t let up? Is there something in there that is still in need of repair? Does it just need time? Do I have to live with this for the rest of my life?” All interesting questions. All questions that my chronic pain patients ask me. I chose to answer them in order of easiest to hardest.

Do I have to live with this for the rest of my life?

No. This is not acceptable. Even if I can’t find the solution, I will help you find someone who can.

Is there something in there that is still in need of repair?

Not likely. Surgeons are usually very thorough, and all of your follow up scans show that everything looks good. Besides, even if something did appear damaged in your knee, it would not likely be a reliable predictor of pain. This means that knee damage does not necessarily equal knee pain.

Does it just need time?

Maybe. The last phase of the healing process (maturation and remodeling) can take up to two years to run its course. This phase involves scar tissue remodeling. Often after a surgery or injury, people may feel stiff or sore for some time. This is because scar tissue builds up during the second phase (proliferation) of healing, but does not function the same as normal tissue. Over time, with stretching and exercise, the scar tissue will break down somewhat, and become more like the surrounding tissue. (See the picture below)


How come this darn pain in the knee won’t let up?

This was the toughest question to answer. He had normal range of motion and strength. He was moving better each day. But for some reason this pain was not responding to treatment the way that we would expect it to. Then a thought occurred to me…phantom limb pain.

Phantom limb pain is a condition very commonly described in amputees. If an individual has a foot or leg amputated, they may describe feelings of pain in that leg, despite the fact that it is not there. Some describe an itch in the big toe. Others will stand up to walk and fall abruptly because they feel as though the normal leg is there and forget that it is not. This occurs because of processes taking place in the brain.

Post central gyrus

The above picture shows the primary sensory cortex (aka post central gyrus) of the brain highlighted in red. This is where all of the somatosensory information (touch, temperature, itch, danger signals, etc.) is processed.


This picture shows the distribution of sensory information throughout the sensory cortex. This is what we would call the sensory homunculus. While there are minor differences between people, this will be mostly the same distribution for everyone.

Homoncular Man

This is the Homuncular Man. It is the brains representation of the body, and is based on the primary sensory cortex. The brain determines what is “normal” based on this body image.

What does this have to do with phantom limb pain? When a limb is amputated, the brain no longer receives information from the nerves of the leg as expected. Because this does not match the brain’s virtual body image, the brain assumes this means something is wrong. In some cases the brain will chose to create pain in the leg’s distribution in response. In other cases, the brain may just assume the leg is still there, giving the person the sensation of a normal leg.

You may have felt something similar to this before. If you normally wear a watch and then forgot to wear it one day, your wrist may feel strange, or may feel like you are still wearing the watch. This is because the watch becomes incorporated into your brain’s virtual body image. When the watch is gone, the brain assumes the watch must still be there, and gives you the sensation that it is. Unfortunately, for many amputees, the brain assumes something must be wrong, and generates pain to attempt to protect the body.

This related to total joint replacement because the ends of the bones are essentially amputated during the surgery, then replaced with metal and plastic.


“How can I have pain in metal? I don’t have any nerves in the metal do I?” he asked me. “No you don’t. You have pain in that area because the brain is giving you pain. The brain’s virtual body is distorted because of this surgery, and that is threatening to the brain. Think of it like this: the mechanic (the surgeon) fixed your piston (your knee) but the warning light on your dash is still on,” I replied. “So it’s a problem with the warning light then!” he said. “Exactly. Now we need to teach the brain a different way,” I told him.

I proceeded to take him through laterality training, imagined movements and mirror therapy and we are now making progress again on his knee pain. The key is to update the brain’s version of the virtual body to include the knee prosthetic. We can do this by thinking about the knee, how it moves, what it should feel like, and what the metal prosthesis should feel like. There is a great article over at NOI that describes this here. This is no simple task, but with some time it can be done.

I have been thinking more about this with my other total joint replacement patients. Is their pain cause by tissue tightness, scar tissue or other factors, or is it potentially a phantom limb like situation. So many patients who undergo total knee replacement describe pain for a prolonged period after surgery. Maybe we need to think harder about treating the phantom limb pain first.

-Patrick Danielson


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